Introduction The field of low-valent main-group metals has grown into a major research topic in synthetic, inorganic chemistry. Some of these main-group complexes are now widely used as reagents and in small-molecules activation. Prominent examples are Jones’s Mg-Mg Dimer 1, which has grown into a state-of-the-art reducing agent in synthetic chemistry, and Roesky’s monomeric beta-diketiminate-Al(I) complex 2, who’s reactivity towards small-molecules is still heavily investigated almost 20 years after its initial synthesis (Figure 1). Neutral and anionic low-valent main-group elements mostly react as nucleophiles or reducing agents due to the driving force of the electropositive metal to convert into the highest oxidation state. Yet, aiming to achieve redox catalysis with main-group elements, a reversibility of these oxidation processes and re-formation of the low-valent species is highly desirable.
Figure 1: Examples for low-valent main-group complexes. Anions for 3 and 4 omitted for clarity.
What is the scope of the cationic complexes? One way to stabilize the low-valent species (at the cost of nucleophilic reactivity) is the use of heavier main-group metals, where the s2-lone pair is energetically lower owing to d- or f-block contraction. Hence, a few examples of reductive eliminations at tin-complexes have been published. Another method to stabilize main-group metals in their lower oxidation state is the preparation of cationic complexes stabilized by soft and weak ligands. Synthesis of such complexes demands for work in pseudo gas-phase environment achievable with the use of weakly coordinating anions such as [B(C6F5)4]- and [Al(ORF)4]- (ORF = C(CF3)3) and weakly coordinating solvents (e.g. fluorinated aromatics). Text-book examples are Jutzi’s Si(II) cation [SiCp*][B(C6F5)4] 3  as well as Krossing’s Ga(I) arene complex [Ga(PhF)2-3][Al(ORF)4] 4 (Figure 1). In particular the latter complex beautifully illustrates the stabilization of the low-valent species: [Ga(PhF)2-3]+ was shown to be accessible via reductive elimination of dihydrogen from an in situ generated [(PhF)2GaH2]+ cation in fluorobenzene. In contrast, neutral Ga(I) complexes readily add dihydrogen in an oxidative addition. Notably, the hexamethylbenzene (HMB) complex of Ga+ retains some nucleophilicity as shown by its protonation with [HPPh3][pf] to form [H-Ga(PPh3)(HMB)]([pf])2. These observations can be explained by the significant lowering of the s2-lone pair energy as a result of the cationic charge at the metal. Consecutively, nucleophilic reactivity of the low-valent metal in [SiCp*]+ and [Ga(PhF)2‑3]+ cations is diminished and the complexes rather react as π-type Lewis acids. Yet, exchange of the soft and weak ligands with more electron-donating ligands activates the lone-pair again to allow for oxidative additions or formation of clusters. However, the clever choice of the applied ligand should allow for a fine-tuning of the lone-pairs energy and stability of the low-valent species, potentially achieving oxidative addition as well as reductive elimination reactions at the same cationic main-group as suggested by the DFT calculated sequence in Scheme 1.
Scheme 1: Computed gas phase thermodynamics for reaction of low-valent gallium cations towards dihydrogen depending on donor-strength of the ligands. All calculations performed at pbe0-d3bj/def2tzvpp//bp86-d3bj/def2svp-level of DFT (density functional theory). The reductive elimination reaction has already been observed experimentally by Wehmschulte et al. The Ga+-carbene complex is literature-known.
An accessible low-valent Al complex salt: While for the group 14 metals as well as Ga, In and Tl a large variety of cationic low-valent complex have been prepared in recent years, a readily available low-valent aluminium complex was hitherto unknown. With the inherent higher reactivity of aluminium’s lone-pair compared to its heavier analogous and the substantial availability of aluminium in the earth’s crust, our group worked on the synthesis of a low-valent aluminium cation for the last 20 years. Here, only an elusive Al cluster could be synthesized in 2000 via a multi-step procedure in low yields from meta-stable AlBr solutions. In recent years, we intensively investigated the direct oxidation of aluminium metal with a large variety of newly developed oxidant salts such as Ag[pf], [HMB][pf] and [PhenazineF][pf]. However, oxidation reactions were only observed upon addition of hard ligands such was acetonitrile and therefore, after initial oxidation, yielded Al(III) cations in a disproportionation. Hence, we thought of a different synthetic approach starting from the first isolated molecular Al(I) complex [(AlCp*)4] 5 prepared by Schnöckel in 1991, which forms an Al4 tetrahedron in solid-state as well as in solution. In 2013, a multi gram-scale synthesis starting from AlCl3, LiAlH4 and KCp* made [(AlCp*)4] widely available. In our study, we present the abstraction of a Cp*-ligand by Li[pf] to form the complex salt [Al(AlCp*)3]+[pf]- 6. In this complex, significantly shortened Al+-AlCp* bonds are observed in the molecular structure whereas a break-up of the bonds between neighbouring AlCp* atoms compared to the Al4 tetrahedron in the starting material is observed. Hence, these observations show the high electrophilicity of the unique Al+ atom in the complex. Analysis of the complex by means of EDA-NOCV support the structure analysis, since the dominating covalent interactions in the complex were determined to represent the electron donation from the s2-lone pairs at the AlCp* atoms into the empty p-orbitals at the unique Al+ atom. Interestingly, this electron donation from the AlCp* units is sufficient to retain some nucleophilicity of the lone-pair at the unique Al atom. This shows in the observation of dimerization of [Al(AlCp*)3]+ to the dication [Al2(AlCp*)6]2+ in solid-state and reversibly in solution. Moreover, addition of Lewis bases to [Al(AlCp*)3]+ results in weakening of the Al+-AlCp* bonds accompanied with a reformation of the AlCp*-AlCp* bonds. The extent of these bond-length changes can be tuned with the donor strength of the added ligands. Here, the molecular structure of the dimethylaminopyridine-substituted cationic Al4+ cluster shows an inversion of bonding lengths. Moreover, the complex readily decomposes in solution, potentially via an extremely reactive [Al(dmap)3]+ cation. Hence, these results show that a suitable strongly electron-donating ligand can potentially abstract an Al+ cation from the cluster. With a readily available cationic low-valent complex at hand, we will study its reactivity towards small molecules and its potential as starting material for the synthesis of new Al+ complex in the future.
Figure 2: Structure of Schnöckel’s [(AlCp*)4] and molecular structures of cations in [Al(AlCp*)3][pf] and [(dmap)3Al(AlCp*)3][pf]. Hydrogens and anions omitted for clarity. Thermal displacement of the ellipsoids was set at 50 % probability
In conclusion, we think that field of low-valent main-group metal-cations is highly promising to yield compounds combining Lewis-acidity and nucleophilicity, which potentially allow for redox-cycling. Yet, this research is only starting now and new starting materials as well as suitable neutral ligands to activate the low-valent cations will be studied intensively by our group in the next years.
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